Broad Line Regions behind a Haze II: A Three-dimensional Electron-scattering Model of the Broad Br$γ$ Line in NGC 3783

Abstract

We investigate the broad Br$γ$ line in the type-1 active galactic nucleus NGC 3783 using three-dimensional Monte Carlo electron-scattering radiative transfer with SKIRT. Paper-I reproduced the observed smooth core and broad wings by convolving an intrinsic Keplerian profile with the analytic electron-scattering kernel of Laor (2006). Here we instead transport photons through explicit source and scattering geometries. For a partially covered Keplerian source, three-dimensional transfer broadens only the scattered photons and leaves an unscattered double-peaked core. For the thin Keplerian source motivated by Paper-I, a comparison of four scattering-geometry classes shows that neither a disk-like scatterer nor the tested halo variants reproduce both the core and wings. We therefore construct a two-component-source (2CS) model containing an inner Keplerian source disk, a rotating disk haze, a nearly spherical halo haze, and direct emission from a slower outer disk. A reference 2CS calculation gives Xi^2 =1.154 at i=15 deg. Polar cones with an input radial optical depth tau_hole=0.1 preserve the line profile and give a measured Thomson-thin sightline. The preferred low redistribution width, full-column ionization, and weak recombination emission cannot all be realized by a uniform thermal halo. The 2CS model is therefore a physically motivated parametric description whose medium must ultimately be tested with self-consistent radiative transfer, ionization, and dynamics.